The term "proof" or "proved goods" always meant roughly 50 percent alcohol. That was considered "right" whiskey. As measurement technology improved, exactly 50 percent alcohol by volume (abv) was considered 100 percent "proof," i.e., 100 proof. Prior to Prohibition, the distiller's goal was to distill out, barrel and bottle his whiskey at as close to "proof" as possible. When government regulation came in and the federal government set up the rules for giving a whiskey its stamp of authenticity, the whiskey had to be at least 100 proof to get that stamp. In those days, i.e., Jack's day, Jack was 100 proof.
After Prohibition, most brands came back at 100 proof but tastes had changed and consumers wanted a lighter taste. Dilution was one way to achieve it and the producers were happy to sell people water for the same price as whiskey. The law set 80 proof as the floor and some brands went there immediately. Others chose intermediate proofs. Some offered a choice, a Bottled-in-Bond, 100 proof expression and a lower proof expression.
Jack was one of the last major brands to resume production after Prohibition because Tennessee retained state prohibition for several years after National Prohibition was repealed, so I'm not sure if it came back at 100 proof or 90 proof, but it was 90 proof until the last time the Federal Excise Tax (FET) was increased, which was October 1, 1985. A number of brands, including Jack, took a proof decrease then instead of taking a price increase to cover the increased tax. That's when Jack went to 86 proof.
Every distiller who talks about their "family recipe, unchanged for 200+ years" is BSing you to some extent, because many aspects of production are different now than they were a century or two ago, as you might expect. The "recipe," in terms of the proportions of the different grains, may be the same, they may even be using the same or a similar yeast strain, but proof of distillation, proof of entry (into the barrel) and bottling proof have all changed.
Mostly, it should be noted, American whiskies have changed for the better. Because the process was so hit-or-miss in the old days, there was a lot of bad whiskey produced. Now most of it is pretty good.